Kinetics of stipe gravitropism in the mushroom fungus Coprinus cinereus under the conditions of microgravity simulation provided by clinostat treatment.

The extent of the gravitropic response, measured as the angle of the stipe apex at maximum curvature, was dependent upon the gravitational exposure time. The reaction time did not depend on exposure time, and interruption of gravitational exposure by a period of clinostating allowed the gravitational stimulus to decay. It is concluded that the gravitropic impulse is an 'all-or-nothing' signal in Coprinus cinereus, that perception and response probably occur in the same tissue regions, and that sustained exposure to the unidirectional gravity vector is necessary for the normal gravitropic response. The presentation time was found to be 7 min. Immediately after reaching curvature, stipes placed on the clinostat after various gravity exposure times 'relaxed' by 5 degrees. This relaxation suggests that gravitropic bending is a two-stage process with an initial, reversible, phase of plastic bending followed by a 'fixation' phase providing the gravitropic stimulus has been maintained.